I wouldn't move or fertilize it until it's done blooming. They don't like their roots disturbed at all, so if you plant it in another pot be careful not to disturb the root ball. How large is it? One of mine has been in the same pot for a long time too, not sure what to tell you on how long it can stay in the same size. Mine seems to do fine rootbound. Sorry I can't help much, I'm sure someone that knows more will answer soon.

It should have a good root ball by two years of age, so repotting now shouldn't be a problem. Growers push Bougainvillea with regular fertilizer all during the growing season, but don't use too much nitrogen or it will be mostly leaves and not flowers. In southeast Asia, many people grow Bougainvillea as a bonsai plant in very small containers, and root prune every year to keep it in the same container indefinitely. This seems to work better in tropical conditions with regular fertilizing, as under American conditions Bougainvillea will tend to sulk if not fed and underpotted long term.

A 3-gal pot will suit a bougie for a long time, repotting is best right before active new growth in spring, but can be done anytime successfully, make the transfer to the new pot quickly to avoid breaking the fragile roots, water the roots in, and set it aside somewhere warm but not too sunny for 7-10 days to adjust, and then you can resume normal care.

Its the fragile roots that pose a problem, if you are quick yet gentle, shock should be minimal.